Plant fibers containing cellulose such as hemp, cotton, jute, wood pulp or bamboo have long been used as naturally-derived, renewable raw materials in various applications such as paper, clothing and packaging materials. Cellulose is a polysaccharide that does not normally exist in molecular form, but rather adopts a multilayer structure in the form of cellulose microfibrils consisting of aggregates of cellulose molecules, microfibril bundles consisting of aggregates of cellulose microfibrils, and aggregates thereof.
In recent years, a technology has been developed for obtaining cellulose nanofibers by fibrillating plant fibers containing cellulose to the nanometer level by subjecting to mechanical and/or chemical treatment, and these cellulose nanofibers are attracting attention as resin composite materials having low specific gravity and high strength (see Patent Document 1).
Since cellulose nanofibers per se are extremely highly hydrophilic as a result of containing a large number of hydroxyl groups, they are normally obtained by subjecting plant fibers containing cellulose to mechanical treatment in an aqueous medium. However, since cellulose nanofibers obtained after fibrillating to the nanometer level in water are highly hydrophilic and a network is formed by the cellulose nanofibers, the viscosity and water retention of the resulting slurry become extremely high. Consequently, there was the problem of requiring an extremely long drainage time when dehydrating by filtration and the like an aqueous dispersion of cellulose nanofibers.
In addition, even after having taken the time to obtain cellulose nanofibers by draining, if the resulting cellulose nanofibers end up drying, they end up aggregating due to hydrogen bonding between the nanofibers, thereby resulting in the problem of it being difficult to uniformly re-disperse despite having comparatively superior dispersibility in water. Consequently, it is even more difficult to form a composite of highly hydrophilic cellulose nanofibers in highly hydrophobic resins. For example, although a method is known for compounding cellulose nanofibers and resin by dehydrating sheet-like cellulose nanofibers by solvent displacement and the like prior to drying followed by impregnating in the resin (see Patent Document 2), in this method, since the cellulose nanofibers in the resin are limited to those in the form of a sheet, it was difficult to uniformly disperse the nanofibers in the resin.